This invention relates to a control system for motor graders and the like having a circle mounted blade or moldboard, and more particularly to a control system for adjusting the position of the circle mounted blade about the axis of the circle, which system includes means for automatically exerting a holding force between the circle and the circle mounting structure to thereby hold the circle and blade supported thereby against movement during grading operations, thus enabling a significant improvement in the smoothness and accuracy of the grade surface produced by the motor grader.
It will be understood that during grading operations it is often necessary to adjust the angle of the blade of the motor grader with respect to the direction of travel of the motor grader depending upon the width of the strip of surface to be graded and upon the amount of material picked up by the blade at a particular point in the grading operation. Thus, the operator of the motor grader may make adjustments in the angle of the blade with respect to the direction of travel of the motor grader by rotating the circle upon which the blade is mounted about its axis. The prior art control systems for enabling the adjustment of the angle of the blade with respect to the direction of motion of the motor grader have seriously reduced the accuracy and smoothness of the graded surface which could be produced due to the working clearances and tolerances required between the blade supporting circle member and the mounting structure therefor as well as the normally expected wear on the mating surfaces of the blade supporting circle member and its mounting structure.
It will be understood that the vertical positioning of the blade with respect to the surface to be graded is established by other control means of the motor grader and that any change in such vertical positioning due to the operation of control means for adjusting the angle of the blade with respect to the direction of travel of the motor grader will reduce the smoothness and accuracy of the resulting grade surface. Modern motor graders use an automatic blade control (A. B. C.) device for reading the desired grade level from a reference grade line or wire and automatically adjusting the vertical positioning of the blade to maintain the reference level. However, such devices cannot compensate for changes in the vertical positioning of the blade due to wear in the mating surfaces of the blade support structure and any changes in the vertical positioning of the blade which may occur during adjustment of the angle of the blade with respect to the direction of motion of the motor grader will result in departures from the reference grade level. Since tolerances as small as 1/8 inch are now required in road grading operations, it will be seen that departures from the reference grade level during the adjustment of the angle of the blade with respect to the direction of motion of the grader, or the working tolerances and wear inherent in the mounting structures which enable such adjustment of the blade, have made prior art control systems for such adjustment impractical or undesirable.